Diastereoselective aldol reactions of furaldehyde using a chiral boronate as auxiliary: Application to the synthesis of enantiomerically pure and highly functionalized 2,3-disubstituted furanyl alcohols

The aldol reactions of various ketene silyl acetals of lithium enolates with furaldehyde 2, which bears a chiral boronate group at the furan C-3 position, as auxiliary have been studied. It was found that (R) diastereoselectivity was more favorable than (S) diastereoselectivity and moderate diastereoselectivity was achieved. Some of the resulting aldol diastereomers were chromatographically separable by simple flash column chromatography on silica gel, leading to optically pure aldol adducts. The absolute stereochemistry of the aldol adducts were determined by X-ray crystallographic analysis. Further transformation of the carbon-boron bond to a carbon-carbon bond was achieved in a Suzuki coupling reaction to furnish highly functionalized and enantiomerically pure 2,3-disubstituted furyl alcohols. One of the furyl alcohols was allowed to rearrange to hydroxypyranone in order to demonstrate the usefulness of this methodology. KGaA, 69451 Weinheim, Germany, 2003.